Help with Crazy Idle

[JerseyS14]

Hey guys, yes i did seach but i couldn't find something exactly like what i'm experiencing.

Problem: (96 240sx auto)

My car runs fine and idles fine... BUT my RPM needle jumps constantly anywhere from 1000 to past 8000. It has no affect on my driving being that the needle does not reflect the actual idle that the vehicle is idling at.

From what i read so far, it might be a sensor on my transmission (idle speed sensor or TPS, i forgot) or possibly just a bad tachometer.

If any of you guys have info on this, your help would be appreciated. If it is a sensor on my transmission that i can check the wiring, please let me know where i can find the sensor (left or right side, front or rear end).

I am pretty knowledgable in mechanics and I've had this 96 240sx for just over a week (proud new S14 owner YaY!)

Thanks in advance,

John

[240sxFTW]

so is it a rough idle or is it just the tach? my guess is that you must have a bad ground with the tach? i dont know anything about electrical. Is the rest of your cluster in proper working order? If it actually has a rough idle then clean out your iacv.

[JerseyS14]

Yes its just the RPM needle that does this and everything else on the tach works fine. The car does not run rough at all. The RPM needle just has a mind of its own.

Say when i'm at idle (stopped at a light), my engine is at idle and its smooth but the needle can be jumping up to like 4-5k on its own witch no affect on the actual rpm state of the vehicle (the engine will still be at around 600-800rpms)... This does not only happen when at a light, it happens from when i start the car to when i turn it off. I'll be on the highway and it can be way past my redline.

Maybe for like 2-3 minutes at a time a few times a day, my needle will actually reflect what my engine is really running at.

Makes it seem like a bad ground or connection like you said but i hae no idea where to start looking. Should i pull out the tach or maybe its a sensor on the engine or transmission that i could check.

[ZOMG.SR20]

its a loose wire connecting to your tach/cluster

[JerseyS14]

I pulled out the tach (I'm actually still sitting in the car) and checked the wires coming from the harness. I unplugged each connector 1 at a time to see which one turned the rpm gauge off and found it... all wires were in tact (no loose/frayed wires) and even the ground was fine. i tried running an extra ground to it just to see if it made a difference and it still didn't change anything... now i'm confused.... maybe new tach?

well if i can't find anything at least i changed my instrument bulbs to Red.

[Triperformance]

Next step is to get a EWD (eletrical wiring diagram) and see what affects that part of the circuit before everyone else guess's doing the trial and error method till you find it lol. Best of luck and keep us posted on your progress.

[JerseyS14]

You know as soon as i put everything back in and took a drive... it worked perfectly fine for a good 10 minutes though not perfect cause it was just a little shakey but not jumping like it was. I was just hoping someone had the same exact problem and could let me know what i should look at or fix...